actions-scheduled-workflow-example: fix cron spec (#5572)

The prior cron spec would trigger 4 times a day, at 05:00, 05:30, 17:00, and 17:30.  The `*/30` is "any minute divisible by thirty" which is twice per hour.

So fix the spec to use `30` in place of `*/30` to match the description.

Author: philpennock

data/reusables/repositories/actions-scheduled-workflow-example.md

@@ -6,6 +6,6 @@ This example triggers the workflow every day at 5:30 and 17:30 UTC:
 on:
   schedule:
     # * is a special character in YAML so you have to quote this string
-    - cron:  '*/30 5,17 * * *'
+    - cron:  '30 5,17 * * *'
 
 ```